The ability to detect and identify trace quantities of analytes has become increasingly important in virtually every scientific discipline, ranging from part per billion analyses of pollutants in sub-surface water to analysis of cancer treatment drugs in blood serum. Raman spectroscopy is one analytical technique that provides rich optical-spectral information, and surface-enhanced Raman spectroscopy (SERS) has proven to be one of the most sensitive methods for performing quantitative and qualitative analyses. A Raman spectrum, similar to an infrared spectrum, consists of a wavelength distribution of bands corresponding to molecular vibrations specific to the sample being analyzed (the analyte). In the practice of Raman spectroscopy, the beam from a light source, generally a laser, is focused upon the sample to thereby generate inelastically scattered radiation, which is optically collected and directed into a wavelength-dispersive spectrometer in which a detector converts the energy of impinging photons to electrical signal intensity.
Among many analytical techniques that can be used for chemical structure analysis, Raman spectroscopy is attractive for its capability to provide rich structure information from a small optically-focused area or detection cavity. Compared to a fluorescent spectrum that normally has a single peak with half peak width of tens of nanometers to hundreds of nanometers, a Raman spectrum has multiple bonding-structure-related peaks with half peak width of as small as a few nanometers.
Typically, the Raman signatures of a sample have been obtained using a spectrometer as shown in FIG. 1. A Raman spectroscopy apparatus typically includes a light source, a spectrometer, and a detector. A typical light source for Raman spectroscopy is a laser. The laser beam can be directed toward the sample to produce Raman scattering, which is a form of non-elastic scattering of incoming photons by molecules within the sample. The Raman scattered light is directed toward a spectrograph which allows the analysis of the wavelength components of the incoming light. Typically, a dispersive spectrometer (such as a Czerny-Turner spectrometer) or a Fourier-transform spectrometer can be used. The spectrometer is connected to a detector, for example, a charge-coupled-device, which converts the incoming photons to electrons. The converted electrons can be read out by an electrical circuit for further storage, display, or analysis.
Other components can be used to improve the performance of the Raman spectroscopy apparatus. For example, a laser line filter (LF) can be used to block light generated by the laser except for the light at the desired wavelength. A dichroic mirror (DM) can be used to separate the excitation light and the Raman scattered light. The dichroic mirror either reflects the laser beam and transmits the Raman scattered light, or transmits the laser beam and reflects the Raman scattered light (the configuration in FIG. 1 is an example of the former). A microscope objective (MO) focuses the laser beam to allow excitation of a small region of the sample, and improves the collection of the Raman scattered light. The sample (S) can be placed on a stage for positioning. A bandpass filter (BF) can be used to block the laser beam from entering the spectrograph. Finally, mirrors (shown as a line) can be used to steer the laser beam or the Raman scattered light.
A spectrometer provides the Raman spectrum of a sample all across a given wavelength.